package riivo.shortespath.util;

import java.util.LinkedList;

public class ThreadPool extends ThreadGroup {

  private boolean isAlive;

  private LinkedList taskQueue;

  private int threadID;

  private static int threadPoolID;

  /**
   * Creates a new ThreadPool.
   * 
   * @param numThreads
   *            The number of threads in the pool.
   */
  public ThreadPool(int numThreads) {
    super("ThreadPool-" + (threadPoolID++));
    setDaemon(true);

    isAlive = true;

    taskQueue = new LinkedList();
    for (int i = 0; i < numThreads; i++) {
      new PooledThread().start();
    }
  }

  /**
   * Requests a new task to run. This method returns immediately, and the task
   * executes on the next available idle thread in this ThreadPool.
   * <p>
   * Tasks start execution in the order they are received.
   * 
   * @param task
   *            The task to run. If null, no action is taken.
   * @throws IllegalStateException
   *             if this ThreadPool is already closed.
   */
  public synchronized void runTask(Runnable task) {
    if (!isAlive) {
      throw new IllegalStateException();
    }
    if (task != null) {
      taskQueue.add(task);
      notify();
    }

  }

  protected synchronized Runnable getTask() throws InterruptedException {
    while (taskQueue.size() == 0) {
      if (!isAlive) {
        return null;
      }
      wait();
    }
    return (Runnable) taskQueue.removeFirst();
  }

  /**
   * Closes this ThreadPool and returns immediately. All threads are stopped,
   * and any waiting tasks are not executed. Once a ThreadPool is closed, no
   * more tasks can be run on this ThreadPool.
   */
  public synchronized void close() {
    if (isAlive) {
      isAlive = false;
      taskQueue.clear();
      interrupt();
    }
  }

  /**
   * Closes this ThreadPool and waits for all running threads to finish. Any
   * waiting tasks are executed.
   */
  public void join() {
    // notify all waiting threads that this ThreadPool is no
    // longer alive
    synchronized (this) {
      isAlive = false;
      notifyAll();
    }

    // wait for all threads to finish
    Thread[] threads = new Thread[activeCount()];
    int count = enumerate(threads);
    for (int i = 0; i < count; i++) {
      try {
        threads[i].join();
      } catch (InterruptedException ex) {
      }
    }
  }

  /**
   * A PooledThread is a Thread in a ThreadPool group, designed to run tasks
   * (Runnables).
   */
  private class PooledThread extends Thread {

    public PooledThread() {
      super(ThreadPool.this, "PooledThread-" + (threadID++));
    }

    @Override
    public void run() {
      while (!isInterrupted()) {

        // get a task to run
        Runnable task = null;
        try {
          task = getTask();
        } catch (InterruptedException ex) {
        }

        // if getTask() returned null or was interrupted,
        // close this thread by returning.
        if (task == null) {
          return;
        }

        // run the task, and eat any exceptions it throws
        try {
          task.run();
        } catch (Throwable t) {
          uncaughtException(this, t);
        }
      }
    }
  }
}
